Aerosolized tobramycin (TOBI) has been shown to be safe and efficacious in cystic fibrosis (CF) patients over six years of age in helping to control the chronic pseudomonas aeruginosa frequently associated with CF pulmonary disease. Phase III studies using 300 mg of aerosolized TOBI documented a mean peak serum concentration of 1.0 micrograms/mL in CF patients older than six years of age. While aerosolized aminoglycosides offer an attractive option for anti-pseudomonal therapy in young children, systemic absorption of an aerosol dose in young children cannot be predicted from data in older subjects. This multi-center study proposes to assess the peak serum tobramycin level after escalating doses of aerosolized TOBI in children less than six years of age. The first cohort of 10 subjects (ages 6 months to 6 years) received a single, 180 mg dose of TOBI. Based on a mean peak serum tobramycin concentration of 0.48 micrograms/mL and absence of significant side effects, a second cohort of 20 subjects was studied using a single 300 mg dose of TOBI. Results from this cohort are pending. There were only 9 subjects studied at this site (Children's Hospital, Cincinnati). The plan is to use information gained from this Phase I study as the basis for future multi-center studies evaluating safety and efficacy of TOBI in children with CF under six years of age.